Brooksella: what are star cobbles?

Back in the far-off year of 2012, when I was helping to compile instances of paleontological type specimens found in National Park Service units, we had to make decisions about various edge cases. One of these was how to handle names for what later turned out to be pseudofossils. We decided to record the information as historically relevant but did not include the "taxa" in any counts. On this blog we've actually covered a couple of them already, "Lingula calumet" and "Paradoxoides barberi" from within or very near Pipestone National Monument. Another is "Brooksella canyonensis", a putative jellyfish from the Proterozoic Nankoweap Formation of Grand Canyon National Park. It was first reported as such in Van Gundy (1937) and then named, not entirely enthusiastically, in Bassler (1941). "B. canyonensis" has fared poorly as a jellyfish, but has had its supporters as an organic feature (e.g., Glaessner 1969; Kauffman and Steidtmann 1981; Kauffman and Fursich 1983; tentatively Ciampaglio et al. 2006). However, I favor an inorganic interpretation. Admittedly, there are several to choose from: gas-escape structures or compaction (Cloud 1968), "sand-volcano"-type fluid escape (Ford and Breed 1977; Ford 1990), and mud rolls (Fedonkin and Runnegar 1992).

"B. canyonensis" was not the first species in the genus Brooksella, though. Brooksella was named by Charles Walcott for "star cobbles" from the Coosa Valley of Alabama (Walcott 1896), now attributed to the middle Cambrian-age Conasauga Formation (Nolan et al. 2023). In fact, he named three taxa for different forms of cobbles: B. alternata, B. confusa, and Laotira cambria (Walcott 1896). Star cobbles got their name because at their best they look like the stereotypical twinkly pointed things you might doodle. Some of them even have five rays, although six is more typical and they are more lobed than pointed, so it's not a perfect match.

Brooksella (A–D, K) and Laotira (E–H, J) as illustrated by Walcott (1898) and reproduced as Figure 1 in Nolan et al. (2023) (which see for full caption). CC BY 4.0.

Walcott interpreted the objects as representing jellyfish, which are probably not the first thing you think of when fossils come to mind, but jellyfish fossils are in fact known elsewhere. In this case, though, the interpretation hasn't proved tremendously popular over time, and numerous alternatives have been proposed. These alternatives, though, generally involve some kind of organic origin, either as a true body fossils or a trace fossil of some sort. It's not hard to see why: they look like something that *ought* to be organic, even if the identity of that something is unclear. (Anyone who has gone out fossil hunting will probably recognize this feeling. Sometimes you're right, sometimes you're wrong.)

Nolan et al. (2023) have published a detailed reassessment of Alabama Brooksella. As part of it, they prepared a lovely supplemental figure of various hypotheses, with thumbnail evaluations (discussed at greater length in the text). (*Warning*: Hold off on clicking the link if you'd rather not get their solution immediately.) Studies of Brooksella from the past couple of decades have interpreted it as a trace fossil (either a feeding burrow or a coprolite) or a glass sponge (hexactinellid). Nolan et al. subjected star cobbles to about as many tests as can legally be done to rocks in their analysis of the various possibilities, and came to several conclusions, including:

• Brooksella specimens do not have a sponge's anatomy. There aren't spicules, features previously interpreted as ostia (pores) bear a strong resemblance to pitting left behind when lichen are cleaned off, and lobes do not feature opening at their ends for radial canals (which were also not found).
• The orientation of the specimens when found in situ was with the putative central osculum (excurrent vent) down in the sediment, which is an inconvenient place for an osculum. Furthermore, many examples did not even have an "osculum".
  
• The specimens include internal voids and tubes, but these spaces do not correspond to the external form, unlike primary burrows (although this does not preclude the specimens having "captured" parts of burrows that were passing through). Furthermore, the internal features do not include common burrowing structures such as backfill.
  
• The specimens have the same composition as silica concretions from the same rocks, and are very comparable overall, with the same kind of weathering rings, lichen pitting, and random internal voids and tubes.
  

Nolan et al. concluded that Brooksella is no different from the local concretions except for the lobes, and should therefore "be considered a pseudofossil until proven otherwise." A consequence of this conclusion is that Brooksella, not being a glass sponge, would not have been a source of silica for preservation of fossils in the Conasauga. (It's not stated, but it seems that it would have been a sink instead.) It further goes to show that you shouldn't trust strange things in the Cambrian.

Brooksella (A–E) and concretions (F–K) collected from the Conasauga Formation by Nolan et al. (scale bar 1 cm, or 0.4 in); Figure 5. CC BY 4.0.

References

Bassler, R. S. 1941. A supposed jellyfish from the pre-Cambrian of the Grand Canyon. Proceedings of the United States National Museum 89(3104):519–522.

Ciampaglio, C. N., L. E. Babcock, C. L. Wellman, A. R. York, and H. K. Brunswick. 2006. Phylogenetic affinities and taphonomy of Brooksella from the Cambrian of Georgia and Alabama, USA. Palaeoworld 15:256–265.

Cloud, P. E., Jr. 1968. Pre-metazoan evolution and the origins of the Metazoa. Pages 1–72 in E. T. Drake, editor. Evolution and environment. Yale University Press, New Haven, Connecticut.

Fedonkin, M. A., and B. N. Runnegar. 1992. Proterozoic metazoan trace fossils. Pages 389–395 in J. W. Schopf and C. Klein, editors. The Proterozoic biosphere: A multidisciplinary study. Cambridge University Press, Cambridge, United Kingdom.

Ford, T. D. 1990. Grand Canyon Supergroup: Nankoweap Formation, Chuar Group, and Sixtymile Formation. Pages 49–70 in S. S. Beus and M. Morales, editors. Grand Canyon geology. Oxford University Press, New York, New York.

Ford, T. D., and W. J. Breed. 1977. Chuaria circularis Walcott and other Precambrian fossils from the Grand Canyon. Journal of the Palaeontological Society of India 20:170–177.

Glaessner, M. F. 1969. Trace fossils from the Precambrian and basal Cambrian. Lethaia 2(4):369–393.

Kauffman, E. G., and F. Fursich. 1983. Brooksella canyonensis: A billion year old complex metazoan trace fossil from the Grand Canyon. Abstracts with Programs - Geological Society of America 15(6):608.

Kauffman, E. G., and J. R. Steidtmann. 1981. Are these the oldest metazoan trace fossils? Journal of Paleontology 55:923–947.

Nolan, M. R., S. E. Walker, T. Selly, and J. Schiffbauer. 2023. Is the middle Cambrian Brooksella a hexactinellid sponge, trace fossil or pseudofossil? PeerJ 11:e14796. doi:https://doi.org/10.7717/peerj.14796.

Van Gundy, C. E. 1937. Jellyfish from Grand Canyon Algonkian. Science 85(2204):314.

Walcott, C. D. 1896. Fossil jelly fishes from the Middle Cambrian Terrane. Proceedings of the United States National Museum 18:611–614.

Walcott, C. D. 1898. Fossil Medusæ. U.S. Geological Survey, Washington, D.C. Monograph 30.